A 120 volt circuit, 100' in length, #12 AWG Cu THHN, serving a single outlet with a cord and plug load of a Treadmill driven by a Computer controlled Variable Frequency Drive. When this treadmill is in use by the most fit users it will trip the circuit breaker which is a Square D QO 20 Ampere. It does this at Several minutes into the workout with the user running at nine MPH at a 10% slope. The first circuit ran through a converted panel cabinet were the neutral bar is still in use with all of the neutrals through the cabinet connected to this same buss bar. Yes I know that this is a code violation. The service equipment change out was done by an "open" shop as the low bid contractor. About thirty feet of this circuit is run through two inch flex (FMC) containing approximately thirty conductors. I will get a precise count tomorrow and add it to this post as an edit.
At the clients direction, based on the vender's assertion that the treadmill was checking out fine and it had to be the circuit, I built a brand new set of circuits. These circuits is in the same raceway and I used the existing spares that are included in the wire count. As a first pass I provided four separate twenty ampere circuits so that each piece of powered exercise equipment will be on it’s own circuit but three of those circuits are part of a multiwire branch circuit. The neutrals for both new circuits; the multiwire and the single circuit have been separated from the neutral bar and spliced through. There have still been intermittent trips but they are fewer. It seems important to note that none of the intermittent overloads that have occurred since the new circuits were energized have happened while more than one machine is in use. I do not know if that was true before. So in spite of the use of a multiwire branch circuit to provide the dedicated outlets the circuit is functioning as a simple branch circuit for trouble shooting purposes.
Given the table ampacity of the #12 THHN Cu after applying the derating for four to six current carrying conductors the portion of the raceway run that serves only these outlets is within it’s code ampacity. I base that conclusion on the thirty ampere ampacity of #12 THHN Cu at thirty amps times 80% yielding an ampacity of 24 amperes. During the period in question there have been no elevated ambient temperature issues by actual measurement. The voltage drop on the new circuits does not exceed five percent at twenty amperes when measured with a Ideal Sure Test©. The voltage drop with the machine in use is even less when measured with a Fluke 89 meter. Current measurement hovers around ten amperes using two different digital ammeters with intermittent runs of 23 and 25 amps that are very brief. We are not seeing any over current that last for more than fifteen seconds at a time. Even with the same users we cannot replicate the problem.
There are already #10 THHN Cu spares available in the two inch FMC so it is an easy matter to sub those in to deal with derating issues but if a more accurate conductor count reveals that I need to drop to a forty percent ampacity then I will run the necessary number eight’s. The circuits are not carrying heavy loads at present so I doubt that will prove curative.
If you were in my position what other steps might you take to exonerate the building wiring and get the ball back in the exercise vender's court.